Śatapatha Brāhmaṇa ("of a hundred paths") 13.3.2

Now this (Sacrificer), having- conquered by means of the supreme Stoma--the Katushtoma, the Krita among dice,--on the next day establishes himself on the Ekavimsa, as a firm foundation: from the Ekavimsa, as a firm foundation, he subsequently ascends to the next day, the seasons; for the Prishtha (-stotras) are the seasons, and the seasons are the year: it is in the seasons, in the year, he establishes himself.

The Sakvarî (verses) are the Prishtha (-stotra of the second day): there is a different metre for each (verse), for different kinds of animals, both domestic and wild ones, are immolated here on each (day). As to the Sakvarî (verses) being the Prishtha, it is for the completeness of the horse (sacrifice); and different kinds of animals are immolated on different (days), because different stomas are performed on the different (days of the Asvamedha).

As to this they say, 'These--to wit, goats and sheep and the wild (beasts)--are not all animals; but those--to wit, the bovine (victims)--are indeed all animals.' On the last day he immolates bovine (victims), for they--to wit, bovine (victims)--are all animals: he thus immolates all animals. They are sacred to the All-gods, for the completeness of the horse, for the horse is sacred to the All-gods. They are many-formed (or, many-coloured), whence animals are many-formed; and they are of distinct forms (or colours), whence animals are of distinct forms.